The present invention relates generally to the measurement of radioactivity, and, more particularly, to the measurement of radioactivity in certain radionuclides characterized by a decay pattern in which there is emission of two quanta of radiation, either gamma or x-rays, in coincidence or with essentially no delay between them.
Two important radionuclides producing coincident pairs of quanta are iodine-125 and cobalt-60, both of which are widely used in medicine. Iodine-125 has a relatively short half-life of approximately 60 days, which makes it well suited for use as a tracer in radioimmunoassay procedures for the measurement of levels of specific antigens or antibodies in the blood. Cobalt-60 is an isotope commonly used as a source in radiation therapy. In some radioimmunological studies, it is necessary to know the absolute amount of iodine-125 present in the sample being measured. Likewise, it is extremely important from a safety standpoint to be able to determine the absolute source strength of a cobalt-60 source used in radiation therapy.
One type of instrument commonly used to detect radiation of the aforedescribed type is a scintillation detector including a scintillation crystal and a photomultiplier tube. The crystal is adapted to receive the radionuclide while the phototube is designed to detect scintillations produced in the crystal by quanta emitted from the radionuclide. The scintillations detected by the phototube are converted into electrical pulses having pulse heights corresponding to the energy of the scintillations being detected.
When radionuclides produce pairs of quanta of radiation practically in time coincidence, some scintillations or events are detected which are the result of single quanta, the other quanta of the pair either being masked or not contributing to the event as detected. Such events are referred to as "single events". Other scintillations or events are detected which are the result of a coincidence of a pair of quanta. Such events are referred to as "coincident events" and give rise to pulses which are the sum of the energy responses produced by each quanta in the pair producing the event. Therefore, the pulse height or energy spectrum resulting from detection of such a radionuclide includes a pair of peaks resulting from the detection of single events and a single peak of relatively high energy resulting from the detection of coincident events. In the case of iodine-125, the energy of each quanta of each pair is substantially or exactly the same. Accordingly, for iodine-125 the energy spectrum includes one single event peak and one coincident event peak.
It has been pointed out by researches using multi-channel analyzers in combination with scintillation detectors that the source strength of a sample of radionuclide producing coincident pairs of quanta can be determined mathematically from the total number of counts resulting from single events and the total number of counts resulting from coincident events. In this regard, a multichannel analyzer, as the term is generally understood, has a capability of segregating detected pulses by pulse-heights and providing a count of pulses for each of a relatively large number of fixed pulse-height increments or channels. In effect, a multichannel analyzer has the capability of producing a histogram equivalent to the energy spectrum resulting from the decay of a radionuclide being tested. Accordingly, with a multichannel analyzer, the researcher determines source strength (S) by totaling the counts in all channels within the single event peaks (N.sub.S) and by totaling the counts in all channels within the coincident event peak (N.sub.C) and by applying the equation ##EQU1##
In practice, multichannel analyzers are very complicated and expensive instruments. Also, they provide the researcher with much more information than needed in measuring the radioactivity of test samples. Therefore, multichannel analyzers are not used for routine experimental measurement of radioactivity. Rather, the common practice is to use a two step procedure. First, a sample of the type of radionuclide which will subsequently be analyzed in test samples is standardized by determining its absolute source strength in a multichannel analyzer. Then, the standardized sample is transferred to a scintillation counter which will be used in subsequent measurements of the source strengths of various test samples. In this regard, it is well known that scintillation counters are never totally effective in counting the absolute number of radioactive disintegrations of a sample since a significant portion of the radiation does not reach or is not measurable by the detector. Therefore, any given scintillation counter has a counting efficiency the value of which depends upon many factors including the detector and its geometry relative to the sample being measured as well as the electrical parameters of the counter. Consequently, before the scintillation counter can be employed in the subsequent measurements of radioactive test samples, the counting efficiency must be obtained using the standardized sample. This is accomplished by counting events in a measurement channel in the counter for a predetermined period of time. The counting efficiency then becomes a ratio of the number of counts per unit time divided by the source strength of the standardized sample. Having obtained the counting efficiency for the counter in the measurement channel, the scintillation counter then may be utilized to determine the source strength of other radioactive test samples simply by counting events in the measurement channel and dividing the count per unit time by the counting efficiency.
In the foregoing two step procedure, the relatively short half-life of iodine-125 poses serious problems. In particular, the source strength changes so rapidly that frequent calibration of the standardized source is required. With a multichannel analyzer such frequent calibration is expensive and time consuming.
To avoid the frequent recalibration of iodine-125 source, some have employed a standardized sample of iodine-125 having a strength known at a particular time and have attempted to compute the current source strength based upon the best known half-life value of iodine-125. One difficulty with this approach is that the half-life of iodine-125 is not known to a high degree of accuracy. In fact, the uncertainty of the calculated current source strength of the sample increases exponentially with increasing age of the sample.
Still another approach which has been employed in an attempt to avoid frequent recalibration of iodine-125 source has been to use simulated or mock sources which have decay properties similar to iodine-125 but which have a longer half-life. One can obtain a mock source such as iodine-129 which has been calibrated against a known standard iodine-125 source. The mock source then can be used to approximate the counting efficiency of a scintillation counter for subsequent use in measuring test samples including iodine-125. Clearly, this approach is inherently prone to error since the mock source can never precisely simulate the decay properties of iodine-125 and can only be used to approximate the counting efficiency of the scintillation counter.
It should also be mentioned in passing that the trend in governmental regulations controlling the use of radioactive substances for medical purposes is to require more frequent and more accurate calibration of the radioactive sources and instruments for detecting radioactivity. It is expected that a typical requirement soon to be imposed will be that radioactive sources and instruments be accurately calibrated at least daily.
In view of the foregoing problems, it should be apparent that there is a significant need in the field of radiation detection for a convenient technique for accurately determining the source strength of radionuclides such as iodine-125 and cobalt-60, and for directly determining the counting efficiency of instruments used to measure radiation from radionuclides of this type. The present invention clearly fulfills this need.